1. Field of the Invention
The present invention relates to a semiconductor laser element, and in particular to an InAlGaP-based semiconductor laser element.
2. Description of the Related Art
Currently, POFs (plastic optical fibers) are receiving attention in the field of the broadband home networks, in the industrial fields (for transmission of various signals), and the like. Although the transmission rates through the POFs are currently approximately 100 Mbps, there are demands for further increase in the transmission rate in order to increase the amount of transmitted information. In addition, since InAlGaP-based semiconductor laser elements emit light in the 650 nm band, and the POFs exhibit low loss in the 650 nm band, the InAlGaP-based semiconductor laser elements are considered to be used as light sources with the POFs.
U.S. Pat. Nos. 5,345,463 and 6,798,808 disclose structures for improving the high-speed modulation characteristics of InAlGaP-based semiconductor laser elements. In the disclosed structures, a first p-type cladding layer is formed on a semiconductor active layer, an n-type current-blocking layer having an opening is formed on the first p-type cladding layer, and a second p-type cladding layer is formed in the opening so that a current-confinement region is realized by a backward-bias structure formed with the n-type current-blocking layer and the second p-type cladding layer. In addition, a p-type contact layer is formed on the current-confinement region, and the semiconductor active layer is realized by, for example, a laminated structure formed of a first optical guide layer, an i-type multiple-quantum-well active layer, and a second optical guide layer.
The diffusion coefficient of zinc as a p-type dopant is greater in the InAlGaP-based material than in the As-based material such as the AlGaAs-based material, so that there is a tendency that the p-type dopant can readily diffuse in a crystal during crystal growth or heat treatment of the p-type cladding layer. However, if the p-type dopant diffuses to the semiconductor active layer, the p-type dopant can produce non-radiative recombination centers in the active layer, so that the characteristics of the semiconductor laser element deteriorate, for example, the lifetime of the semiconductor laser element is reduced. U.S. Pat. No. 5,345,463 proposes formation of an antidiffusion layer (which is undoped or has a low carrier concentration) in a vicinity of the active layer during formation of the first p-type cladding layer in order to prevent diffusion of the p-type dopant into the semiconductor active layer.
In addition, the InAlGaP-based material has greater electric resistance and thermal resistance than the AlGaAs-based material and the like, so that there is a tendency that the InAlGaP-based semiconductor laser element requires higher driving current and generates greater heat than the semiconductor laser element made of AlGaAs-based material and the like. Further, U.S. Pat. No. 6,798,808 proposes high concentration doping in the p-type cladding layers for lowering the element resistance of the semiconductor laser element.
Although the optical guide layers are also made of an InAlGaP-based material, doping in the optical guide layers has not been proposed for the following reasons, which are indicated in U.S. Pat. No. 6,798,808.
(1) Conventionally, it has been considered that when the optical guide layer is doped, the function of an optical guide layer preventing diffusion of the dopant from the p-type cladding layers into the quantum-well active layer deteriorates.
(2) Conventionally, it has been considered that when the dopant diffuses from the p-type cladding layers into the optical guide layer, and the carrier concentration in the optical guide layer becomes high, defects produced in the optical guide layer by the diffused dopant become non-radiative recombination centers, and the characteristics of the semiconductor laser element deteriorates, since a greater portion of light is distributed in the optical guide layer than in the p-type cladding layers.
For the above reasons (1) and (2), conventionally, it has been considered that deterioration in the characteristics of the semiconductor laser element becomes more serious (i.e., the lifetime of the semiconductor laser element is reduced) when the carrier concentration in the optical guide layer becomes higher.
As described above, in the structure disclosed in U.S. Pat. No. 6,798,808, the p-type cladding layers are doped with high concentration dopant. Although this technique is effective in reducing the element resistance of the semiconductor laser element, the effect is insufficient. Therefore, further reduction in the element resistance of the semiconductor laser element is required.